Carotenoids represent one of the most widely distributed and structurally diverse classes of natural pigments, producing pigment colors of light yellow to orange to deep red. Eye-catching examples of carotenogenic tissues include carrots, tomatoes, red peppers, and the petals of daffodils and marigolds. Carotenoids are synthesized by all photosynthetic organisms, as well as some bacteria and fungi. These pigments have important functions in photosynthesis, nutrition, and protection against photooxidative damage. For example, animals do not have the ability to synthesize carotenoids but must obtain these nutritionally important compounds through their dietary sources.
Industrially, only a few carotenoids are used for food colors, animal feeds, pharmaceuticals, and cosmetics, despite the existence of more than 600 different carotenoids identified in nature. This is largely due to difficulties in production. Presently, most of the carotenoids used for industrial purposes are produced by chemical synthesis; however, these compounds are very difficult to make chemically (Nelis and Leenheer, Appl. Bacteriol. 70:181-191 (1991)). Natural carotenoids can either be obtained by extraction of plant material or by microbial synthesis; but, only a few plants are widely used for commercial carotenoid production and the productivity of carotenoid synthesis in these plants is relatively low. As a result, carotenoids produced from these plants are very expensive. One way to increase the productive capacity of biosynthesis would be to apply recombinant DNA technology (reviewed in Misawa and Shimada, J. Biotech. 59:169-181 (1998)). Thus, it would be desirable to produce carotenoids in non-carotenogenic bacteria and yeasts, thereby permitting control over quality, quantity and selection of the most suitable and efficient producer organisms. The latter is especially important for commercial production economics (and therefore availability) to consumers.
Structurally, the most common carotenoids are 40-carbon (C40) terpenoids; however, carotenoids with only 30 carbon atoms (C30; diapocarotenoids) are detected in some species. Biosynthesis of each of these types of carotenoids is derived from the isoprene biosynthetic pathway and its five-carbon universal isoprene building block, isopentenyl pyrophosphate (IPP). This biosynthetic pathway can be divided into two portions: 1) the upper isoprene pathway, which leads to the formation of farnesyl pyrophosphate (FPP); and 2) the lower carotenoid biosynthetic pathway, comprising various crt genes which convert FPP into long C30 and C40 carotenogenic compounds. Both portions of this pathway are shown in FIG. 1.
Typically, the formation of phytoene represents the first step unique to biosynthesis of C40 carotenoids (FIGS. 1 and 2). Phytoene itself is a colorless carotenoid and occurs via isomerization of IPP to dimethylallyl pyrophosphate (DMAPP) by isopentenyl pyrophosphate isomerase (encoded by the gene idi). The reaction is followed by a sequence of 3 prenyltransferase reactions in which geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP) are formed. The gene crtE, encoding GGPP synthetase, is responsible for this latter reaction. Finally, two molecules of GGPP condense to form phytoene (PPPP). This reaction is catalyzed by phytoene synthase (encoded by the gene crtB).
Lycopene is a “colored” carotenoid produced from phytoene. Lycopene imparts the characteristic red color of ripe tomatoes and has great utility as a food colorant. It is also an intermediate in the biosynthesis of other carotenoids in some bacteria, fungi and green plants. Lycopene is prepared biosynthetically from phytoene through four sequential dehydrogenation reactions by the removal of eight atoms of hydrogen, catalyzed by the gene crtI (encoding phytoene desaturase). Intermediaries in this reaction are phytofluene, ζ-carotene, and neurosporene.
Lycopene cyclase (CrtY) converts lycopene to β-carotene. β-carotene is a typical carotene with a color spectrum ranging from yellow to orange. Its utility is as a colorant for margarine and butter, as a source for vitamin A production, and recently as a compound with potential preventative effects against certain kinds of cancers. β-carotene is converted to zeaxanthin via a hydroxylation reaction resulting from the activity of β-carotene hydroxylase (encoded by the crtZ gene). For example, it is the yellow pigment that is present in the seeds of maize. Zeaxanthin is contained in feeds for hen or colored carp and is an important pigment source for their coloration. Finally, zeaxanthin can be converted to zeaxanthin-β-monoglucoside and zeaxanthin-β-diglucoside. This reaction is catalyzed by zeaxanthin glucosyl transferase (encoded by the crtX gene).
In addition to the carotenoid biosynthetic genes and enzymes responsible for creation of phytoene, lycopene, β-carotene, zeaxanthin, and zeaxanthin-β-glucosides, various other crt genes are known which enable the intramolecular conversion of C40 compounds to produce numerous other functionalized carotenoid compounds by: (i) hydrogenation, (ii) dehydrogenation, (iii) cyclization, (iv) oxidation, (v) esterification/glycosylation, or any combination of these processes.
Many of the bacteria within the family Enterobacteriaceae are naturally pigmented, thus indicating the ability of these organisms to produce carotenoids. Furthermore, C40 carotenoid biosynthesis has been particularly well-studied within the genus Pantoea, a small group of organisms previously classified within a broad group of bacteria all formerly known within the genus Erwinia [see Hauben et al., Syst. Appl. Microbiol. 21(3):384-397 (August 1998), for details concerning the reclassification of the large former genus Erwinia into four phylogenetic groups comprised of Erwinia, Pectobacterium, Brenneria gen. nov., and Pantoea]. For example, several reviews discuss the genetics of carotenoid pigment biosynthesis, such as those of G. Armstrong (J. Bact. 176: 4795-4802 (1994); Annu. Rev. Microbiol. 51:629-659 (1997)). Gene sequences encoding crtEXYIBZ are available for Pantoea agglomerans (formerly known as E. herbicola EHO-10 (ATCC #39368)), P. ananatis (formerly known as E. uredovora 20D3 (ATCC #19321)), P. stewartii (formerly known as E. stewartii (ATCC #8200)), and P. agglomerans pv. milletiae (U.S. Pat. No. 5,656,472; U.S. Pat. No. 5,545,816; U.S. Pat. No. 5,530,189; U.S. Pat. No. 5,530,188; U.S. Pat. No. 5,429,939; WO 02/079395 A2; see also GenBank® Accession No.'s M87280, D90087, AY166713, and AB076662, respectively). However, the existing literature provides limited information concerning diversity of gene sequences encoding crtEXYIBZ and the genetic organization of these sequences in organisms that are related to these well-characterized Pantoea species.
The problem to be solved, therefore, is to identify more nucleic acid sequences encoding all or a portion of the carotenoid biosynthetic enzymes from organisms that are related to Pantoea agglomerans, P. ananatis, P. stewartii, and P. agglomerans pv. milletiae, to facilitate studies to better understand carotenoid biosynthetic pathways, provide genetic tools for the manipulation of those pathways, and provide a means to synthesize carotenoids in large amounts by introducing and expressing the appropriate gene(s) in an appropriate host. This will lead to carotenoid production superior to synthetic methods.
Applicants have solved the stated problem by isolating seven unique open reading frames (ORFs) in the carotenoid biosynthetic pathway encoding CrtE, Idi, CrtX, CrtY, CrtI, CrtB and CrtZ enzymes from a yellow-pigmented bacterium designated as Pantoea stewartii strain DC413. The gene sequences and the genetic organization of the gene cluster in P. stewartii DC413 are different from those of the P. stewartii ATCC 8200.